Injection molding is a technology commonly used for high-volume manufacturing of parts made of meltable plastic material, most commonly of parts made of thermoplastic polymers. During a repetitive injection molding process, a plastic resin, most often in the form of small beads or pellets, is introduced to an injection molding machine that melts the resin beads under heat, pressure, and shear. The now molten resin is forcefully injected into a mold cavity having a particular cavity shape. The injected plastic is held under pressure in the mold cavity, cooled, and then removed as a solidified part having a shape that essentially duplicates the cavity shape of the mold. The mold itself may have a single cavity or multiple cavities. Each cavity may be connected to a feed channel by a gate, which directs the flow of the molten resin into the cavity. A molded part may have one or more gates. It is common for large parts to have two, three, or more gates to reduce the flow distance the polymer must travel to fill the molded part. The one or multiple gates per cavity may be located anywhere on the part geometry, and possess any cross-section shape such as being essentially circular or being shaped with an aspect ratio of 1.1 or greater.
A typical injection molding procedure comprises four basic operations: (1) heating the plastic in the injection molding machine to allow the plastic to flow under pressure; (2) injecting the melted plastic into a mold cavity or cavities defined between two mold halves that have been closed; (3) allowing the plastic to cool and harden in the cavity or cavities while under pressure; and (4) opening the mold halves and ejecting the part from the mold.
During the injection molding process, the molten plastic resin is injected into the mold cavity and the plastic resin is forcibly injected into the cavity by the injection molding machine until the plastic resin reaches the location in the cavity furthest from the gate. The resulting length and wall thickness of the part is a result of the shape of the mold cavity.
In some cases, it may be desirous to reduce the wall thickness of injected molded parts to reduce the plastic content, and thus cost, of the final part. Reducing wall thickness using a conventional injection molding process can be an expensive and a non-trivial task. In fact, conventional injection molding machines (e.g. machines injecting molten plastic resin at higher pressures) have a practical limit as to how thin walls of a part may be molded. Generally speaking, conventional injection molding machines cannot mold parts having a thinwall ratio (as defined by an L/T ratio set forth below) of greater than about 200. Furthermore, molding thinwall parts with thinwall ratios of more than 100 requires pressures at the high end of current capability and thus, presses that are capable of handling these high pressures.
When filling a thinwall part, the current industry practice is to fill the mold cavity at the highest possible rate the molding machine can achieve. This approach ensures that the mold cavity is filled before the polymer solidifies or “freezes off” in the mold, and provides the lowest possible cycle time since the polymer is exposed to the cooled mold cavity as quickly as possible. This approach has two drawbacks. The first drawback is that to achieve very high filling velocities requires very high power loads, and this requires very expensive molding equipment. Further, most electric presses are unable to provide sufficient power to achieve these high filling rates, or require very complicated and expensive drive systems that substantially increase the cost of the molding equipment making them impractical economically.
The second drawback is that the high filling rates require very high pressures. These high pressures result in the need for very high clamping forces to hold the mold closed during filling, and these high clamping forces result in very expensive molding equipment. The high pressures also require injection feed systems and mold cores that are made from very high strength materials, typically hardened tool steels. These high strength feed systems and mold cores are also very expensive, and can be impractical economically for many molded components. Even with these substantial drawbacks, the need for thinwall injection molded components remains high, since these components use less polymer material to form the molded part, thereby resulting in material savings that more than offset the higher equipment costs. Further, some molded components require very thin design elements to perform properly, such as design elements that need to flex, or design elements that must mate with very small features of other design elements.
As a liquid plastic resin is introduced into an injection mold in a conventional injection molding process the material adjacent to the walls of the cavity, immediately begins to “freeze,” or solidify, or cure, or in the case of crystalline polymers the plastic resin begins to crystallize, because the liquid plastic resin cools to a temperature below the material's no flow temperature and portions of the liquid plastic become stationary. This frozen material adjacent to the walls of the mold narrows the flow path that the thermoplastic travels as it progresses to the end of the mold cavity. The thickness of the frozen material layer adjacent to the walls of the mold increases as the filling of the mold cavity progresses, this causes a progressive reduction in the cross sectional area the polymer must flow through to continue to fill the mold cavity. As material freezes, it also shrinks, pulling away from the mold cavity walls, which reduces effective cooling of the material by the mold cavity walls. As a result, conventional injection molding machines fill the mold cavity with plastic very quickly and then maintain a packing pressure to force the material outward against the sides of the mold cavity to enhance cooling and to maintain the correct shape of the molded part. Conventional injection molding machines typically have cycle times made up of about 10% injection time, about 50% packing time, and about 40% cooling time.
As plastic freezes in the mold cavity, conventional injection molding machines increase injection pressure (to maintain a substantially constant volumetric flow rate due to the smaller cross-sectional flow area). Increasing the pressure, however, has both cost and performance downsides. As the pressure required to mold the component increases, the molding equipment must be strong enough to withstand the additional pressure, which generally equates to being more expensive. A manufacturer may have to purchase new equipment to accommodate these increased pressures. Thus, a decrease in the wall thickness of a given part can result in significant capital expenses to accomplish the manufacturing via conventional injection molding techniques.
In an effort to avoid some of the drawbacks mentioned above, many conventional injection molding operations use shear-thinning plastic material to improve flow characteristics of the plastic material into the mold cavity. As the shear-thinning plastic material is injected into the mold cavity, the shear rate generated due to the relative motion of the flowing plastic between the stationary mold cavity walls tend to reduce viscosity of the plastic material, thereby allowing the plastic material to flow more freely and easily into the mold cavity. As a result, it is possible to fill thinwall parts fast enough to avoid the material completely freezing off before the mold is completely filled.
Reduction in viscosity is directly related to the magnitude of shear rates generated due to the relative motion of the flowing plastic material and the stationary mold cavity walls. Thus, manufacturers of these shear-thinning materials and operators of injection molding systems have been driving injection molding pressures higher in an effort to increase shear, thus reducing viscosity. Typically, high output injection molding systems (e.g., class 101 and class 30 systems) inject the plastic material in to the mold cavity at higher melt pressures more. Manufacturers of shear-thinning plastic material teach injection molding operators to inject the plastic material into the mold cavities above a minimum melt pressure. For example, polypropylene resin is typically processed at pressures greater than 6,000 psi (the recommended range from the polypropylene resin manufacturers, is typically from greater than 6,000 psi to about 15,000 psi). Press manufacturers and processing engineers typically recommend processing shear thinning polymers at the top end of the range, or significantly higher, to achieve maximum potential shear thinning, which is typically greater than 15,000 psi, to extract maximum thinning, and to achieve better flow properties from the plastic material. Shear thinning thermoplastic polymers generally are processed in the range of over 6,000 psi to about 30,000 psi. Even with the use of shear thinning plastics, a practical limit exists for conventional injection molding of thin walled parts. This limit is currently in the range of thinwall parts having a thinwall ratio of 200 or more. Moreover, even parts having a thinwall ratio of between 100 and 200 may become cost prohibitive as these parts generally require injection pressures between about 15,000 psi and about 30,000 psi.
High production injection molding machines (i.e., class 101 and class 30 molding machines) that produce thinwalled consumer products exclusively use molds having a majority of the mold and feed system made from high hardness materials. High production injection molding machines typically experience 500,000 cycles per year or more. Industrial quality production molds must be designed to withstand at least 500,000 cycles per year, preferably more than 1,000,000 cycles per year, more preferably more than 5,000,000 cycles per year, and even more preferably more than 10,000,000 cycles per year. These machines have multi cavity molds and complex cooling systems to increase production rates. The high hardness materials are more capable of withstanding the repeated high pressure clamping operations than lower hardness materials. However, high hardness materials, such as most tool steels, have relatively low thermal conductivities, generally less than 20 BTU/HR FT ° F., which leads to long cooling times as heat is transferred through from the molten plastic material through the high hardness material.
Furthermore, the high pressures existing in conventional injection molding machines require very robust feed systems. These feed systems are generally formed from a single piece of strong material to limit the number of seams or joints where molten plastic material could escape due to the high injection pressures. As a result, the feed channels are formed by drilling channels through a single piece of strong material, which greatly limits achievable flow geometries. Any change in flow direction must be formed by drilling channels along different axes, which necessarily meet at a junction that has sharp edges. These sharp edges disproportionately increase the energy required to drive the flow and create significant local increases in shear rate which can increase stress in molded parts. Even in the absence of sharp edges or corners, feed systems may contain constrictions or lengths that contribute to high shear rates.
In general, polymer flow in a feed system is laminar within the flow rates commonly used in the injection molding industry. Turbulent flow is generally understood not to occur within the feed system, even in flow systems with very high velocity and with polymers having relatively low flow viscosity. It is useful, however, to consider the Reynolds Number (Re) as a means of characterizing the influence of flow geometry on flow velocity, and the associated effects on polymer shear. The velocity of a polymer will vary in the feed system as flow encounters turns and changes in channel cross-sectional area. In the case of a corner, or a turn, in the feed system, these Re differences can be represented as a ratio of the faster moving flow portions relative to the slower moving flow portions. In general, reducing this ratio to a minimum value is desirable to achieve lower shear conditions and lower resistance to flow.
Generally, Reynolds Number (Re) is a dimensionless parameter familiar to those skilled in the art, which relates fluid density ρ, viscosity μ, velocity v, and hydraulic diameter of the flow path D as:Re=ρvD/μ. Stability and laminarity of flow are inversely proportional to Re. Typical flows in injection molding feed systems are stable or laminar, however Re will increase locally around sharp corners, bends, edges and in regions of constriction of flow. It is typically these regions that will create the highest shear rate in a feed system, and thus reduction of shear in these regions below critical or maximum permissible shear rates in plastic materials is of significant concern in feed system design. Even with the ever increasing injection pressure ranges of existing injection molding machines, a practical limit remains of about 200 (L/T ratio) for molding thinwalled parts in conventional (e.g. high) variable pressure injection molding machines and thinwall parts having a thinwall ratio of between about 100 and about 200 may be cost prohibitive for many manufacturers.
Additionally, typical single piece drilled feed systems are difficult to clean when changing from one type (or color) of thermoplastic molding material to another. Currently, feed systems are cleaned by draining the system, using a brush of some sort to do a first stage cleaning, and running new molding material to flush out or purge the old material out of the feed system. This process results in considerable down time for the mold and a waste of new (flushing) thermoplastic material.